David Rawlins - Chartered Surveryors

Damp

When considering the issue of damp in a building, a valuer must have reference to the RICS Appraisal of Valuation Standards (Red Book) 5th addition. Clause 6.2.5.of UK Appendix 3 specifies:

Where the valuer reports that works must be carried out to the Property as a condition of any advance, and also has identified the Property as being:

• Of Architectural or historic interest, or listed as such
• In a Conservation Area
• Of unusual construction

Then the report must state that a person with appropriate specialist knowledge be asked to give advice on the appropriate works. In such cases, Valuers should only offer advice themselves if they are sure they are competent to give advice, which, if adopted, would not be detrimental to the properties architectural all historic integrity, its future structural condition, or the conservation of the building fabric.

It goes without saying that all Valuers acting on behalf of lenders believe that they are competent to advise on such issues, irrespective of their general experience & knowledge. One must bear in mind that the training of chartered valuation surveyors is on modern construction rather than pre-1900 buildings and materials. The issue is the difference between cavity and solid construction. This is particularly relevant when considering moisture penetration.

Whilst not now forming part of the current Red Book, the Guidance Notes Appendix 2 effective from 1/5/1996 stated of the following:

2.2 Rising Dampness

• 2.2.1 –Old buildings are generally constructed of solid walls and these rely on their ability to lose moisture by evaporation to maintain reasonably dry conditions. Changes to the building, such as coating with dense or impervious materials, or changes in the vicinity, such as drainage or increased ground levels, can lead to increased dampness. (Further problems can be caused when rising damp is misdiagnosed).
• 2.2.2 Standard treatments for solid walls suffering from rising dampness include chemical injection and the provision of dense renders internally. These can cause severe problems in cob, pise and a clay lump walls, where slumping can occur as a result. In flint-work, the drilling associated with chemical injection can be very difficult and can cause severe disruption. In thick walls of stone rubble or rubble-filled walls, injected damp proofing is usually ineffective unless grouting is undertaken. Buildings constructed of soft stones, such as lias or chalk block or soft bricks, can suffer severe decay of exposed masonry below an inserted damp proof course (DPC) due to the concentration of dampness, salt deposition and frost action in this zone. Timber frame buildings also require special consideration, to ensure that no timber is trapped below the level of any inserted DPC.
• 2.2.3 As already noted, dense renders inhibit evaporation of water from solid walls and are therefore likely to lead to increased levels of dampness in the wall material itself. Moisture entering the wall will be forced to rise higher until an equilibrium is established between absorption and evaporation. Further decay of the materials within the wall is likely and timbers within, or in contact with, the wall could become affected.
• 2.2.4 There are cases where stripping of internal plaster ssociated with damp proofing work has caused the damage or destruction of historically, or artistically important, wall paintings or a painted decoration. Such decoration was common on walls and other surfaces, even in small cottages, in many parts of the country.

2.3 Penetrating dampness

• 2.3.1 As discussed above, solid walls constructed of porous materials need to be able to lose moisture by evaporation. Frequently, this process is interfered with by the use of dense plasters, renders and mortars, or by the use of impervious paints and other coatings. This can lead to moisture becoming trapped within the wall, increasing decay of the building fabric, and deterioration of living conditions.
• 2.3.2 Standard solutions to damp penetration include the provision of dense renders, impervious coatings and silicon treatments. All of these could exacerbate the problem when used on an historic building.

Such comments are clearly still relevant now. They are also backed by the Planning Policy Guidance Note 15 in respect of works to Listed Buildings. This is the Bible used by Conservation officers, although this is due to the replaced by a new system fairly shortly (new legislation is due).

It is argued the natural ability of a solid wall to dry out moisture, by evaporation, should be allowed to continue unhindered. Equally, with a thick stone wall, with a rubble infill, any injected liquid may extend up the wall, rather than provide a suitable “damp-course”. We are also of the opinion that, by preventing the stone from “breathing”, there is potential frost damage. This may well involve “living with” a degree of dampness to the lower walls internally.

Cement mortars are considered inappropriate in walls of solid construction due to the tendency to retain moisture, rather than allow natural moisture evaporation. A consequence can be frost & sulphate damage to the bricks. This can also be encouraged by the installation of a chemical DPC, which concentrates moisture to the base of the wall. Any salts within the wall can continue to attract moisture from the air even when the original cause of the problem has been attended.